For example, a known torque transmission device is disclosed in a Japanese Patent Publication No.5 (1993)-34541. This torque transmission device has 2-speed forward gear and 1-speed reverse gear and applied to an industrial vehicle such as a forklift truck. FIG. 3 shows a schematic view of the above-described torque transmission device applied to the industrial truck. As shown in FIG. 3, driving torque from an engine (not shown) is transmitted to an input shaft through a torque converter. Then one gear as the driving torque is transmitted is determined among the 2-speed forward gear and the 1-speed reverse gear in response to combination of engaging and releasing at each clutch. The driving torque is finally transmitted to an axle through a differential gear, and then the axle is driven. In the torque transmission device shown in FIG. 3, since the torque converter is provided, the vehicle can smoothly start from halt condition.
In the industrial vehicle, stop-and-go movement is frequently repeated, and the vehicle mostly drives at low speed. Therefore, when the torque converter is applied to the industrial vehicle as shown in FIG. 3, it is difficult to improve fuel economy.
With a view to improving the fuel economy, a vehicle having no torque converter between an engine and a transmission and an electric motor as a drive unit, in which an engine-driven state and a motor-driven state are selectively switched depending on a driving state of the vehicle, that is, what we call a hybrid vehicle is disclosed in a Japanese Patent Laid-open Publication 2001-105908. In the hybrid vehicle, a continuously variable transmission is applied as a transmission. However, shift range of the continuously variable transmission is too wide for the industrial vehicle, and the size of a device is enlarged due to the continuously variable transmission.
Additionally, in the torque transmission device shown in FIG. 3, when the torque converter is removed and an electric motor is provided, it is difficult for the vehicle to start smoothly from the halt condition. Further, axial dimension of the torque transmission device is elongated.
The present invention therefore seeks to provide a torque transmission device with a compact body having adequate shift range to an industrial vehicle and capable of improving fuel economy.